Photoliquefaction and phase transition of m-bisazobenzenes give molecular solar thermal fuels with a high energy density

A series of m-bisazobenzene chromophores modified with various alkoxy substituents (1; methoxy, 2; ethoxy, 3; butoxy, 4; neopentyloxy) were developed for solvent-free molecular solar thermal fuels (STFs). Compounds (E,E)-1–3 in the crystalline thin film state exhibited photoliquefaction, the first example of photo-liquefiable m-bisazobenzenes. Meanwhile, (E,E)-4 did not show photoliquefaction due to the pronounced rigidity of the interdigitated molecular packing indicated by X-ray crystallography. The m-bisazobenzenes 1–4 exhibited twice the Z-to-E isomerization enthalpy compared to monoazobenzene derivatives, and the latent heat associated with the liquid–solid phase change further enhanced their heat storage capacity. To observe both exothermic Z-to-E isomerization and crystallization in a single heat-up process, the temperature increase of differential scanning calorimetry (DSC) must occur at a rate that does not deviate from thermodynamic equilibrium. Bisazobenzene 1 showed an unprecedented gravimetric heat storage capacity of 392 J g−1 that exceeds previous records for well-defined molecular STFs.


Synthesis of compounds 14
Synthesis of 3-(4'-hydroxyphenylazo)acetanilide 1) In a 100 mL beaker, 5.4 g of 3'-aminoacetanilide (36 mmol) was dissolved in 13 mL of water and 13 mL of 37% HCl. The mixture was stirred in an ice bath and 2.6 g of sodium nitrite (38 mmol) was added dropwise. The solution was left to stir for 30 min, while in a second 200 mL beaker, 3.3 g of phenol (35 mmol) and 7 g of NaOH (175 mmol) were dissolved in 25 mL of water. The contents of the first beaker were added dropwise to the second beaker and left to stir for 3 days. After neutralizing with 37% HCl, the resulting precipitate was filtered and washed with water. The crude product was purified by column chromatography of silica gel (ethyl acetate/hexane = 2/1, Rf = 0.3) to obtain an orange solid. Yield: 7.1 g, 77%. 1  Synthesis of 3-(4'-hydroxyphenylazo)aniline 1) In a 500 mL round-bottom flask, 7.1 g of 3-(4'-hydroxyphenylazo)acetanilide (28 mmol) was suspended in 350 mL of 10% NaOH (875 mmol) aqueous solution and refluxed at 100 o C for 3 h. The mixture was then cooled to room temperature and the pH was adjusted to 6 with 37% HCl. The resulting precipitate was filtered, washed with water, and dried in vacuo to obtain an orange solid. The compound was sufficiently pure to be used in the subsequent step without further purification. Yield: 5.9 g, 100%. TLC (ethyl acetate/hexane = 2/1, Rf = 0.6).
Synthesis of 1,3-bis(4'-hydroxyphenylazo)benzene 1) In a 200 mL beaker, 5.9 g of 3-(4'-hydroxyphenylazo)aniline (28 mmol) was placed, and 200 mL of water and 10 mL of 37% HCl were added. The mixture was stirred in an ice bath, and 1.9 g of sodium nitrite (28 mmol) in 10 mL of water was added dropwise. The solution was left to stir for 30 min to diazotize the aniline, while in a second 500 mL beaker, 2.6 g of phenol (28 mmol) and 5.6 g of NaOH (139 mmol) were dissolved in 50 mL of water. The contents of the first beaker were added dropwise to the second beaker and left to stir for one night. After neutralizing with 37% HCl, the precipitate was filtered and washed with water. The crude product was purified by column chromatography with silica gel (ethyl acetate/hexane = 1/2, Rf = 0.2) to obtain a brown solid. Yield: 6.1 g, 69%. 1  Synthesis of 1,3-bis((4-methoxyphenyl)diazenyl)benzene (1) and 4-((3-((4-methoxyphenyl)- In a 500 mL three-neck flask, 3.0 g of 1,3-bis(4'-hydroxyphenylazo)benzene (9.4 mmol) and 2.6 g of K2CO3 (19 mmol) were placed. After replacing the air in the flask with nitrogen, 90 mL of dry acetonitrile and 1.3 g of iodomethane (9.4 mmol) were added, and the mixture was refluxed for 5 h.
After removing the solvent, 100 mL of water was added, and the pH was adjusted to 6 with 37% HCl.
After the air in the flask was replaced by nitrogen, 30 mL of dry DMF and 0.28 g of iodoethane (1.8 mmol) were added, and the mixture was stirred at 60 o C for 15 h. After removing DMF under reduced pressure, the product was extracted with DCM, washed with water several times, and dried over = 0.5) to obtain an orange solid. Yield: 0.27 g, 82%. 1 Table S1. DSC data for bisazobenzenes (E,E)-1~4.    Photoillumination was carried out in CD3CN, as shown in Figure S2.    Photoillumination was performed in CD3CN, as shown in Figure S5.   Photoillumination was carried out in CD3CN, as shown in Figure S8.  Photoillumination was performed in CD3CN, as shown in Figure S11.     Table S3. The activation parameters of the Z-to-E thermal isomerization for bisazobenzenes14.
ΔHisom and ΔHc were determined from the exothermic peaks in the first heating scan of the DSC thermograms. These values were calculated as 100% for the Z-isomer contents.